Nanostructures with high aspect ratios have great potential in a great variety of applications. Their small size resulting in quantum confinement, high aspect ratio, unique electrical, optical, mechanical properties, etc makes them highly desirable for applications such as interconnects, electrodes, sensors, nano-scale lasers, etc.
The existence of NSs is well documented in academic literature and is widely investigated. The NSs is reported to be synthesised by numerous techniques. An example is the carbon nanotube where the synthesis technique includes arc discharge, laser vaporisation, electron beam and catalytic pyrolysis. Other known methods are to use chemical vapour deposition (CVD) and plasma enhanced CVD (PECVD). Background information discussing carbon nanotubes is disclosed in prior art document ‘Nanotubes for electronics’ in the December 2000 issue of Scientific American (P. G. Collins et al.) pp. 38-45. This document discloses a method of CNT production, wherein a substrate is placed in a vacuum oven or flow tube, heated to temperatures of the order of 500° C. to 1200° C. and a carbon containing gas such as methane is introduced optionally in the presence of a transition metal-containing catalyst, whereupon it decomposes into, inter alia, carbon vapour. Some of the carbon vapour forms or condenses as carbon nanotubes. The catalytic process is similar to techniques used for synthesizing a wide variety of nanowires of different materials such as silicon nanowires where the catalysis is gallium and the feed gas is silane (S. Sharma and M. K. Sunkara, Nanotechnology 15 (2004) pp 130-134) and GaN nanowires using nickel as the catalyst and gallium and ammonia as the feed gas (F. Sammy, NNIN REU Research accomplishments (2004) pp 112)
The synthesis of these nanostructures has been restricted to high temperatures, usually above 500° C. One example is a technique for growing carbon nanotubes which is disclosed in the International patent application WO 99/65821, wherein a method of forming carbon nanotubes on materials such as glass, silica, quartz and silicon using Plasma Enhanced Chemical Vapour Deposition (PECVD) is disclosed. This ‘hot filament’ PECVD method uses high gas temperatures of between 300° C. and 700° C., so as to deposit carbon nanotubes on, for example, glass having a strain point temperature of 666° C. A heat filament situated above the material directly heats the material on to which the carbon nanotubes are deposited, the heat filament providing the energy required to produce the plasma above the substrate, and therefore provide the mechanism to disassociate the hydrocarbon gas and form carbon nanotubes using a catalyst. The glass onto which carbon nanotubes are deposited can then be used in the production of flat panel displays.
A method and system for controlled patterning and growth of single wall and multi-wall carbon nanotubes are known from U.S. Pat. No. 6,858,197. A substrate is coated with a first layer of a first selected metal and a second layer of a catalyst. Provision of the first layer enhances electrical conductivity associated with the carbon nanotube and also helps prevent lift-off of the catalyst in the second layer from the substrate. The gas temperatures are typically in the range 800-1100° C.
An alternative carbon nanotube fabricating system and method is known from US 2005/0109280. The nanotubes are formed in a substrate supported in a temperature regulated chuck. Immediately after the nanotubes have been formed, a cooling cycle is initiated to cool the back of the substrate.
Current techniques involve substantial substrate temperatures and this imposes severe limitations on application development. Recent research has been focused on moving towards lower synthesis temperature and one example is the technique disclosed in the International patent application WO 03/011755, wherein the making of carbon nanotubes at substrate temperatures down to room temperature is disclosed. The contents of WO 03/011755 are incorporated herein by reference in their entirety.
After the disclosure of WO 03/011755, there have been reports of similar techniques with low substrate temperatures below 300° C. as reported in 2003 issue of New journal of Physics (S. Hofmann, B. Kleinsorge, C. Ducati and J. Robertson) pp 153.1 and 2004 issue of Applied Physics Letter (T. M. Minea, S. Point, A. Granier and M. Touzeau) pp 1244 where both techniques show defective carbon nanotubes which we believe is due to the low temperature.
A further process for direct low-temperature synthesis of carbon nanotubes on substrate material are known from CN 1448334. This document discloses growing the carbon nanotubes directly on a multi-layered substrate. The three metal layers include one active metal catalyst layer sandwiched between one metal carrier layer on the substrate and one covering metal layer. The active metal catalyst is Fe, Co, Ni or their alloy, the metal carrier layer and the covering metal layer may be of Au, Ag, Cu, Pd, Pt or their alloy, and the three layers may be formed through vacuum sputtering, chemical vapour deposition, physical vapour deposition, screen printing or electroplating.
The present invention has been devised and modified to provide an improved low temperature PECVD process for the formation and growth of carbon nanotubes.